This invention relates generally to diffraction gratings and holograms, especially those designed for use as security devices to authenticate documents or objects to which they are attached.
Holograms are becoming widely used on credit cards as security devices to authenticate genuine cards. Similar use of holograms is being made, or proposed to be made, in authenticating certificates of various kinds, as seals for containers to restrict unauthorized entry, and similar applications. Currently, such holograms are embossed onto thin plastic with a reflective layer added, the embossing hologram originally being made in an optical laboratory with laser equipment. The plastic replicated holograms are made of very thin material and attached to the credit card, or other device being authenticated, in a manner that an attempted removal of the hologram destroys it. This reduces the likelihood that holograms for counterfeit documents can be removed from other expired or unused cards or documents.
Holograms which reconstruct images of objects are a preferred form of diffraction grating for security applications because they are harder to make. The specialized skills and extensive equipment that is required to make a hologram create a significant barrier for counterfeiters who attempt to make original holograms from an object scene that resembles that of the security hologram to be simulated.
Effort has been directed to making security holograms in which the object scene is chosen such that any copies that might be made by counterfeiters will not look exactly like the original. However, it is difficult to make such a security hologram from which a copy of it can readily be distinguished by the usual observer from the original. Therefore, it is a primary object of the present invention to provide a diffraction grating and hologram, and methods of making them, from which unauthorized copies thereof are more readily apparent.